Lithographic printing plate precursors which are presently used in the field of small-scale commercial printing include (1) a direct drawing type printing plate precursor having a hydrophilic image-receiving layer provided on a water-resistant support, (2) a printing plate precursor of an electrophotographic light-sensitive material having a photoconductive layer provided on a water-resistant support, the photoconductive layer comprising photoconductive zinc oxide and is converted into a printing plate by undergoing image formation and then desensitizing treatment with a desensitizing solution to render the non-image area hydrophilic, and (3) a printing plate precursor of a silver-halide photographic material having a silver halide emulsion layer provided on a water-resistant support.
With the development of office appliances and the expansion of office automation in recent years, it has been desired in the field of small-scale printing to adopt an offset printing system wherein a lithographic printing plate is directly prepared from a direct drawing type lithographic printing plate precursor (the foregoing (1)) utilizing various image forming means, e.g., an electrophotographic printer, a heat-sensitive transfer printer or an ink jet printer without undergoing any other special treatment for conversion into the printing plate.
A conventional direct drawing type lithographic printing plate precursor comprises a support such as paper having on one surface side an image-receiving layer which is a surface layer provided via an interlayer and on the other surface side a back layer. The interlayer and the backlayer are each composed of a water-soluble resin such as PVA or starch, a water-dispersible resin such as a synthetic resin emulsion, and a pigment. The image-receiving layer comprises an inorganic pigment, a water-soluble resin and a water resisting agent
Examples of inorganic pigment conventionally used include kaolin, clay, talc, calcium carbonate, silica, titanium oxide, zinc oxide, barium sulfate and alumina.
Examples of water-soluble resin used include polyvinyl alcohol (PVA), modified PVA such as carboxylated PVA, starch and derivatives thereof, cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose, casein, gelatin, polyvinyl pyrrolidone, vinyl acetate-crotonic acid copolymer, and styrene-maleic acid copolymer.
Examples of water resisting agent used include glyoxal, initial condensates of aminoplasts such as melamine-formaldehyde resin and urea-formaldehyde resin, modified polyamide resins such as methylolated polyamide resin, polyamide-polyamine-epichlorohydrin adduct, polyamide epichlorohydrin resin, and modified polyamide-polyimide resin.
In addition to the above described ingredients, it is known that a cross-linking catalyst such as ammonium chloride or a silane coupling agent can also be used.
In recent plate-making system using various kinds of printers, it is required for an image-receiving layer of the printing plate precursor to have both hydrophilicity sufficient for preventing the occurrence of stain due to adhesion of printing ink and water resistance as a lithographic printing plate, and sufficient adhesion to oleophilic images formed thereon. Various proposals have been made in order to satisfy the requirement.
For instance, a proposal has been made to improve the hydrophilicity and image adhesion by the application of an image-receiving layer prepared by dispersing zinc oxide, kaolinite and alumina as inorganic pigments together with a water-soluble resin, a water resisting agent and acetic acid whereby zinc oxide reacts with acetic acid to form zinc acetate, and coating the resulting dispersion as described in JP-A-63-54288 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). It is also proposed that the hydrophilicity and water resistance are improved by employing the same image-receiving layer described above except that talc or silica is used in place of alumina and an aluminum-based, zirconium-based or titanium-based metal compound is used as the water resisting agent as described in JP-A-63-166590 and JP-A-63-166591.
Also, in case of employing an electrophotographic printer using a dry toner (PPC coping machine) for plate- making, the toner undesirably adheres to non-image area of the resulting printing plate, which forms background stain on prints, when the printing plate is subjected to printing. In order to overcome the problem, a method for controlling surface roughness of the image-receiving layer to a specific range using an organic pigment such as silica having an average diameter of from 5 to 20 Hm as described in JP-B-6-96353 (the term "JP-B" as used herein means an "examined Japanese patent publication") and a method of using as organic pigment, both silica and alumina sol each having an average diameter of from 5 to 20 pm as described in JP-A-62-157058 are proposed.
Further, as an approach for preventing stain occurrences in the non-image area due to the adhesion of ink and increasing adhesion of an ink image to the image-receiving layer during plate-making using a PPC copying machine or a heat-sensitive transfer printer, a method of using colloidal silica having a particle diameter of not more than 20 nm, a pigment such as calcium bicarbonate and a lubricant such as polyethylene wax emulsion in combination as described in JP-A-6-183164, and a method of using synthetic silica powder having a particle diameter of not more than 20 pm, a colloidal silica having a particle diameter of not more than 50 nm and a hydrophilic polyvinyl alcohol resin as described in JP-B-5-17871, are proposed.
On the other hand, the recent spread of various office automated machines, various computers and peripheral appliances thereof and the development of related technology as described above have made it possible to form an image by compilation using a personal computer or a workstation and to output the digital signal of the image directly on a lithographic printing plate precursor from a printer capable of processing digital signal, thereby preparing a printing plate. Also, as a printer capable of processing digital signal which can provide highly accurate images compared with those hitherto known, a laser printer using a dry toner having a particle diameter of from 7 to 8 gm, an ink jet printer of electrostatic ejection type which ejects oil-based ink in the electrostatic field to form an image, and the like have been developed.
However, direct drawing type lithographic printing plate precursors prepared according to conventional techniques are still insufficient with respect to background stains, reproducibility of highly accurate images, durability of image portions (i.e., press life) and the like.
Under these circumstances, it has been desired to provide a direct drawing type lithographic printing plate precursor capable of producing a large number of prints having highly accurate images free from background stains according to the above described system.